The present invention relates to a power supply device for electrical discharge machining and it is an object of the invention to provide a power supply device for electrical discharge machining at a reduced size and cost.
FIG. 15 shows a conventional wire electrical discharge machining device having a wire electrode 1, a workpiece 2, and upper and lower electric energizers, 3a and 3b respectively, used to slidingly apply electric power, which is supplied from a pulse power supply unit 4 through a power supply cable 5, to the wire electrode 1. Also, the upper and lower electric energizers 3a and 3b are disposed within upper and lower machining liquid injection nozzles 13a and 13b, respectively.
A numerical value control unit 6 drives a machining table. FIG. 15 also shows a machining liquid vessel 11, a machining liquid tank 7, a pump 8 used to supply a machining liquid, and a cooling device 9 for cooling the machining liquid. These parts cooperate together in forming a machining liquid supply system. By means of the machining liquid supply system, the machining liquid is supplied so that it is branched into and stored in a liquid section 12a of the machining vessel 11 and into a liquid section 12b, where the machining liquid is to be supplied to a machining gap formed between the wire electrode 1 and workpiece 2 in which electric discharge is generated.
A bed 14 is used to support the workpiece 2 thereon.
Next, the operation of the above-mentioned conventional wire electrical discharge machining device will be described.
Initially, while injecting the machining liquid 12a to the wire electrode 1, a pulse voltage is applied between the wire electrode 1 and workpiece 2. As a result, in mutually opposed minute gaps between the electrode 1 and workpiece 2, the workpiece 2 melts and flies around due to the vaporization and explosion of the machining liquid 12a and the thermal energy generated when electricity is discharged.
The mutually opposed minute gaps are kept constant, and the relative movements of the wire electrode 1 and workpiece 2 in continuous electric discharge are normally controlled according to a control method in which an X-Y cross table (not shown) is controlled by the numerical value control device 6 that uses a difference between the average machining voltage of a machining gap and a target servo voltage.
If the electric discharge is repeatedly generated and the X-Y cross table is controlled in this manner, then the machining liquid vessel 11, bed 14 and workpiece 2 respectively connected to the X-Y cross table are controlled simultaneously, so that machining grooves are formed successively to thereby be able to machine the workpiece 2 into an arbitrary shape.
FIG. 16 shows the structure of the pulse power supply unit 4 which is used to supply a pulse voltage to the wire electrode 1 through the upper and lower electric energizers 3a and 3b.
FIG. 16 shows a printed substrate 41 on which there are mounted circuit component elements such as resistors 44, capacitor 45, transistors 46, and diodes 47. Heat radiating fins 42 are connected to the printed substrate 41 in order to cool the transistors 46 and diodes 47, which are the main heat generating elements. Fans 43 are used to forcibly air cool the main parts of the circuit component elements in order to enhance the heat radiation efficiency of the heat radiating fins 42, and 48 stands for a box member.
In the pulse power supply unit 4, then, there is a forced air cooling method in which the main heat generating elements of the unit 4, that is, the transistors 46 and diodes 47, are cooled by the fans 43.
In FIG. 16, arrows represent the flow of the air sent from the fans 43.
FIG. 17 is an electric circuit diagram which shows an electrical discharge circuit.
In FIG. 17, a waveform i, which is supplied from a power supply E due to the switching operation of a transistor TRI, is decided according to an equation 1 relating to a circuit inductance L, a resistor R, and a power supply voltage E.sub.1. For this reason, it is necessary to reduce the RL component of a cable used for this purpose and normally a coaxial cable is used or a plurality of cables are connected together in parallel to one another. EQU i=E.sub.1 (1-e.sup.-RT/L)/R Equation 1
FIGS. 18A and 18B are section view of an electric elements storage member (also referred to herein as a gap box) which is disclosed in the Unexamined Japanese Patent Application Publication No. Hei. 4-226834. The electric elements storage member is used to store therein an isolation diode out of an electric power module for supplying a desired machining pulse to a wire electrode and a workpiece. The storage member is disposed adjacently to a machining gap and the wire electrode. FIG. 18A is a longitudinal section view of the gap box, while FIG. 18B is a plane section view thereof.
FIGS. 18A and 18B show coaxial cables 21, conductive elements 22 used to connect the coaxial cables 21 to diode assemblies 23, an insulation plate 24, an electric conduction member 25, a lower wire guide structure 26, an injection conduit 27 used to form a flow passage for feeding an insulating machining liquid not only to a wire guide and but also to a machining liquid injection member 26 which injects a machining liquid to a machining gap, and an elongated hole 28.
The operation of the above-mentioned electric elements storage member, or gap box, will now be described.
An output signal pulse is sent to the gap box 20 through the coaxial cable 21 provided within a lower arm 29 to a pair of diode assemblies 23.
The intermediate portion of each of the diode assemblies 23 includes in the middle portion thereof an isolation diode 23a having an anode connected to the conductive element 22, while the cathode of the isolation diode 23a is connected to an electrically conductive central block.
During the electrical discharge machining time, a machining pulse having a given waveform is supplied through the coaxial cables 21 to the diode assemblies 23. Since the polarity of the machining pulse is negative, the machining pulse signal is allowed to pass through the isolation diodes 23a to the electrically conductive central block. The machining pulse signal flows through the electric conduction member 25 and lower wire guide structure 26 of the gap box 20.
In order to inject the machining liquid to the machining gap and workpiece, the elongated hole 28 is formed so that it extends through the insulation plate 24 and electric conduction member 25 and reaches the wire guide structure 26. The elongated hole 28 forms a cooling mechanism with respect to the isolation diodes 23a and the other electric elements of the gap box 20.
The conventional power supply device for wire electric discharge machining, as described above, includes fins for heat radiation and fans for forced air cooling and is included with the printed substrate within the box member, so that the volume of the power supply device is excessively large.
Because the power supply device is large, it must be disposed apart from the electrical discharge machining gap, so that the power supply cable for wiring must be increased in length.
Further, since the air cooling fans have a low reliability, it is necessary to provide a filter to prevent dust from invading into the interior of the box member.
In order to solve these problems, one method transports the heat of the interior of the closed box member to the outside thereof by use of a heat pipe where it is forcibly cooled by the outside air. However, the effect of reduction in the occupied volume of the power supply device is minimal and the cost of the power supply device is increased.
In an electrical discharge machining device disclosed in the above-mentioned Unexamined Japanese Patent Application Publication No. Hei. 4-226834, the isolation diodes disposed in the interior of the gap box are cooled by flowing the machining liquid into the elongated hole. However, by this method, as shown in FIG. 18B, only the temperatures of the connecting portions of the isolation diodes can be lowered. There is no reduction in the temperatures of the remaining portions of the isolation diodes, in which generation of heat is greatest, resulting in a poor cooling characteristic of the power supply device.
Further, no solution has been provided for a case in which the machining liquid leaks into the isolation diode side due to corrosion of the elongated hole.